The Evolution of the Sciences 



fractures within the solid mass, which allow some 

 rocks to be crushed by the weight of the super- 

 jacent formations. This mechanical work 

 becomes the source of heat. In 1872 Sir Robert 

 Mallet endeavoured to compute its magnitude. 

 His experiments led him to the conclusion that 

 the pulverisation of one cubic metre of rock 

 would produce sufficient heat to melt 300 kilo- 

 grammes of ice. He inferred from this that the 

 annual crushing of one quarter of a cubic kilo- 

 metre of rock would be sufficient to furnish the 

 entire energy of volcanic action at the present 

 day. We quote this explanation chiefly to ex- 

 pose the flagrant impossibilities which militate 

 against the assumption of the earth's complete 

 solidity. How explain on this hypothesis the 

 way in which a purely local action, by transfer- 

 ring materials from within the earth to the sur- 

 face, could result in the production of enormous 

 mountains, or even of entire islands, such as 

 Hawaii or Iceland? Should we not rather 

 expect to find eruptive centres surrounded by 

 gigantic subsidences? For example, the lava 

 erupted by Jokull in Iceland would have pro- 

 duced a cavity of sufficient capacity to sink 



142 



